JP2014019137A - Biaxially oriented polyester film and coat-type magnetic recording tape using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a biaxially oriented polyester film excellent in terms of workability at a high temperature exceeding the glass transition point of the polyester, for example, despite having an extremely flat surface with a surface roughness (RaA) of 1.5 nm or lower.SOLUTION: In a biaxially oriented polyester film of which at least one surface includes 50 ppm or more (mass standard) of inert particles with an average particle diameter of 0.05-0.2 μm, of which the surface roughness (RaA) is 1.5 nm or lower, of which texture index is confined to a range of 80-99.99%, and of which bearing curve fall is confined to a range of 10-30 nm, the thermal shrinkage of the biaxially oriented polyester film along the width direction of the biaxially oriented polyester film at 130°C is 3% or lower.

Description

  The present invention relates to a biaxially oriented polyester film excellent in processability at high temperatures while having a very flat surface, such as a base film of a coating type magnetic recording tape for data storage.

  Polyester films have been used as base films for magnetic recording tapes because they are relatively inexpensive and have excellent mechanical properties. And when using for the base film of a magnetic-recording tape, it is calculated | required that a polyester film has a flat surface without a rough protrusion and a fault. On the other hand, in a coating type magnetic recording tape formed by coating a magnetic layer on a polyester film, a uniform magnetic layer is efficiently produced if the winding property of the base film and handling properties in the coating process are unstable. In other words, it is required that the polyester film contains particles as a lubricant to form protrusions on the surface. These two requirements are contradictory, and in order to satisfy these requirements, Patent Documents 1 to 5 disclose that the catalyst species should be specific in order to reduce surface defects, and particles to be included in the film. A film having a small number of coarse particles and a film having a small surface defect subjected to such a treatment have been proposed. Patent Documents 6 to 7 propose a biaxially oriented polyester film that is excellent in the stability of the original fabric shape and the electromagnetic conversion characteristics as a magnetic recording medium by reducing the undulation component of the base film focusing on the spatial frequency. Has been made.

  However, in recent years, the demand for high-density recording is tremendous. Particularly in the case of a coating type magnetic recording tape such as a data storage having a very high recording capacity, the film described in Patent Documents 1 to 5 described above that has no surface defects, Even the film that is said to have less undulation in 7 can no longer respond sufficiently.

JP 2004-114492 A JP 2003-291288 A JP 2002-36311 A JP 2002-363310 A JP 2002-059520 A JP 2001-341265 A JP 2004091753 A

  The purpose of the present invention is to form a coating film layer with a very flat surface at a high speed, and it is necessary to dry the coating film layer at a higher temperature. While there is a considerable limitation in speeding up, processing at a high temperature such as exceeding the glass transition temperature of polyester while having a very flat surface with a surface roughness (RaA) of 1.5 nm or less The object is to provide a biaxially oriented laminated polyester film having excellent properties.

  As a result of diligent research to solve the above problems, the present inventors have reduced the heat shrinkage rate of the film even if the surface on which the magnetic layer is to be formed is extremely flat. The present inventors have found that a biaxially oriented polyester film having the properties can be provided, and have reached the present invention.

Thus, according to the present invention, at least one surface contains 50 ppm (mass basis) or more of inert particles having an average particle diameter of 0.05 to 0.2 μm, and the surface roughness (RaA) is 1.5 nm or less. A biaxially oriented polyester film having a background index of 80 to 99.99% and a bearing curve drop of 10 to 30 nm,
A biaxially oriented polyester film having a heat shrinkage rate at 130 ° C. in the film width direction of the biaxially oriented polyester film of 3% or less is provided.

  According to the present invention, as a preferred embodiment of the present invention, the polyester has ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a main repeating unit, and the thickness is 2.0 μm or more and 8.0 μm or less. The surface roughness (RaB) of the other surface having at least two layers is 2.0 nm or more and 6.0 nm or less, and a flat surface having a smaller surface roughness when having at least two layers. The ratio (tA / tB) of the thickness tA (μm) of the layer having the thickness tB (μm) of the layer having a rough surface having a larger surface roughness is 0.5 or more and 9 or less. The layer having a large rough surface contains inert particles having a large average particle diameter of 0.01 μm or more with respect to the average particle size of the inert particles contained in the layer having a flat surface having a smaller surface roughness. The ratio (tA / DpB) of the average particle diameter DpB (nm) of the inert particles added to the layer having a rough surface having a larger surface roughness is 10 or more and 30 or less, the film forming direction The heat shrinkage rate at 130 ° C. in the film is 3% or less, the Young's modulus in the film forming direction is 5 GPa or more, and the contained inert particles are spherical silica particles, crosslinked polystyrene particles, silicone particles, silica-acrylic. A biaxially oriented polyester film comprising at least one of the composite particles is also provided, and the biaxially oriented polyester film of the present invention and a magnetic film formed by coating on the surface on the side on which the magnetic layer is formed. A coated magnetic recording tape comprising a layer is also provided.

  The biaxially oriented polyester film of the present invention has excellent surface properties such as having a very flat surface, but also has a low heat shrinkage rate even in a high temperature environment of coating, so that it can be folded and wrinkled when contacting a process roll. There are no problems, workability such as transportability necessary for practical use is provided, and even minute surface defects that cause errors when applied to coated magnetic recording tapes, especially data storage base films, are reduced. Therefore, it is possible to provide data storage having excellent electromagnetic conversion characteristics.

  Hereinafter, the present invention will be described in detail. For convenience of explanation, the film forming direction of the film may be referred to as a mechanical axis direction, a vertical direction, a longitudinal direction, and an MD direction, and a direction orthogonal to the film forming direction and the thickness direction is referred to as a width direction, a lateral direction, Sometimes referred to as TD direction.

  As the polyester in the present invention, a known polyester can be adopted as long as it can be formed into a film. For example, an aromatic polyester obtained by polycondensation of a diol component and an aromatic dicarboxylic acid component is preferable. Examples of the aromatic dicarboxylic acid component include terephthalic acid, isophthalic acid, 2,6-naphthalenedicarboxylic acid, 4,4′-diphenyldicarboxylic acid, 6,6 ′-(ethylenedioxy) di-2-naphthoic acid, and the like. 6,6 ′-(alkylenedioxy) di-2-naphthoic acid. Examples of the diol component include ethylene glycol, 1,4-butanediol, 1,4-cyclohexanedimethanol, and 1,6-hexanediol.

  Among these, ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate is the main repeating unit from the viewpoint of dimensional stability during processing at high temperature, and ethylene-2,6-naphthalene is particularly preferable. Those having carboxylate as the main repeating unit are preferred. The term “main” as used herein means preferably 60 mol% or more, 70 mol% or more, 80 mol% or more, and more preferably 90 mol% or more.

  Further, from the viewpoint of further improving the dimensional stability against environmental changes, the 6,6 ′-(ethylenedioxy) di-2-naphthoic acid component described in International Publication No. 2008/096612 pamphlet, 6,6 ′-( 6,6 ′-(alkylenedioxy) di-2-naphthoic acid components such as trimethylenedioxy) di-2-naphthoic acid component and 6,6 ′-(butylenedioxy) di-2-naphthoic acid component The thing copolymerized is also mentioned. The copolymerization amount of the (alkylenedioxy) di-2-naphthoic acid component is preferably in the range of 5 to 40 mol%, more preferably in the range of 6 to 35 mol%, particularly 7 to 7 mol, based on the number of moles of the total dicarboxylic acid component. It is in the range of 30 mol%. In the case of copolymerizing the 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component, an ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate component and 6,6 ′-(alkylenediene) The total amount of the (oxy) di-2-naphthoic acid component is preferably 90 mol% or more of the total acid component.

  When the polyester in the present invention does not contain a 6,6 ′-(alkylenedioxy) di-2-naphthoic acid component, it is 6,6 ′-(alkylenedioxy) di- in o-chlorophenol at 35 ° C. When the 2-naphthoic acid component is contained, the intrinsic viscosity when measured in a mixed solvent of P-chlorophenol / 1,1,2,2-tetrachloroethane (40/60 weight ratio) at 35 ° C. is 0. It is preferably 40 dl / g or more, more preferably 0.40 to 1.0 dl / g. If the intrinsic viscosity is less than 0.4 dl / g, cutting may occur frequently during film formation, or the strength of the product after forming may be insufficient. On the other hand, when the intrinsic viscosity exceeds 1.0 dl / g, productivity during polymerization is lowered.

  The melting point of the polyester in the present invention is preferably 200 to 300 ° C, more preferably 210 to 290 ° C, and particularly preferably 220 to 280 ° C. If the melting point is less than the lower limit, the heat resistance of the biaxially oriented film may be insufficient, and if the melting point exceeds the upper limit, the temperature during melt kneading becomes very high, which tends to cause thermal degradation.

  The polyester in the present invention is further copolymerized with other copolymer components known per se within a range not impairing the effects of the present invention, for example, 10 mol% or less, further 5 mol% with respect to the number of moles of repeating units. Copolymerization may be carried out in the following range, and other thermoplastic resins and the like may be blended in a range of, for example, 20% by weight or less, and further 10% by weight or less.

  By the way, the biaxially oriented polyester film of the present invention can be produced from the above-mentioned polyester. However, while maintaining the processing characteristics such as winding and transporting within a practically practical range, the electromagnetic conversion characteristics when data storage is used are advanced. From the viewpoint of maintaining the magnetic layer, the surface on the side on which the magnetic layer is formed contains particles having an average particle diameter of 0.05 to 0.2 μm, based on the mass of the film layer forming the surface, at 50 ppm or more. The roughness (RaA) needs to be 1.5 nm or less. When the average particle size of the inert particles contained is smaller than this range, or when the content is small, the transportability is deteriorated, and scratches called scratches are likely to enter the film, and the error rate and dropout are deteriorated. Invite. In addition, when the average particle diameter and the surface roughness (RaA) exceed these ranges, the surface becomes too rough when used for a data storage base film having a high recording density such as a storage capacity of 3 TB or more. Conversion characteristics will deteriorate. A preferred average particle size range is 0.06-0.2 μm, more preferably 0.08-0.18 μm. Moreover, the range of preferable content is 55 ppm or more, More preferably, it is 60 ppm or more, More preferably, it is 65 ppm or more, Most preferably, it is 70 ppm or more. The preferable range of the surface roughness (RaA) is 1.4 nm or less, more preferably 1.3 nm or less.

  As the inert particles to be contained, particles that do not contain coarse particles or contain very few particles are preferable. Therefore, particles that are easy to have a sharp particle size distribution curve and are likely to exist in the form of primary particles are preferable. Organic polymer particles such as silicone particles, crosslinked acrylic resin particles, crosslinked polyester particles, and crosslinked polystyrene particles, and spherical silica particles It is preferably at least one particle selected from the group consisting of composite particles of silica and organic polymer, and in particular, a group consisting of silicone particles, crosslinked polystyrene particles and spherical silica particles, and silica-acrylic composite particles It is preferably at least one kind of particle selected from Of course, when these particles are contained, it is preferable to filter with a filter, to treat the surface of the particles with a dispersant, or to enhance kneading with an extruder in order to eliminate coarse particles.

  By the way, it is preferable that the relative standard deviation of the particle diameter of all the particles when the particle size distribution curve of the particles is viewed is 20% or less, and further 15% or less. From such a viewpoint, it is preferable to have a single peak when viewing the particle size distribution curve. Whether or not there is a single peak is determined by creating a particle size distribution curve with the particle diameter on the horizontal axis and the particle frequency on the vertical axis, and when the measurement pitch of the particle diameter on the horizontal axis is 0.01 μm, there is only one peak. Even if there are a plurality of peaks, it means that there is no dent that is 50% or less with respect to the height of the lower peak between the peaks.

  Below, the manufacturing method of a polyester film is demonstrated. First, the polyester production method of the present invention includes, for example, a first reaction in which an aromatic dicarboxylic acid or an ester-forming derivative thereof and an alkylene glycol are esterified or transesterified to synthesize a polyester precursor, and the precursor It consists of a second reaction in which the product is polycondensed, and a method known per se can be adopted.

  The preferable first reaction condition may be performed under normal pressure, but it is preferable to perform the reaction under a pressure of 0.05 MPa to 0.5 MPa because the reaction rate can be easily increased. Moreover, it is preferable to perform the temperature of 1st reaction in the range of 210 to 270 degreeC. By making the reaction pressure within the above range, it is possible to suppress the generation of by-products typified by dialkylene glycol while facilitating the progress of the reaction. At this time, the alkylene glycol component is preferably used in an amount of 1.1 to 6 moles relative to the acid component present in the reaction system in which the first reaction is carried out from the viewpoint of maintaining the reaction rate and the physical properties of the resin. More preferably, it is 2-5 mol times, More preferably, it is 3-5 mol times.

  Further, in order to increase the reaction rate of the first reaction, it is preferable to use a catalyst known per se, for example, a metal compound having a metal component such as Li, Na, K, Mg, Ca, Mn, Co, and Ti. Among these, when performing under pressure, Mn and Ti compounds are preferable from the viewpoint of easy progress of the reaction. In particular, a Mn compound is preferable because the dispersibility of the inert particles to be contained is easily improved.

  The amount of catalyst to be added is preferably in the range of 10 to 150 mmol%, more preferably 20 to 100 mmol%, especially 30, in terms of metal elements, based on the number of moles of all acid components present in the first reaction. It is preferable to be in the range of ˜70 mmol% because the reaction rate can be promoted, the formation of coarse insoluble foreign matters due to the catalyst can be suppressed, and the heat resistance of the resulting copolymerized aromatic polyester can be maintained at a high level. In addition, when adding a titanium compound, it is preferable to add so that it may exist from the time of the esterification reaction start of a 1st reaction, and as above-mentioned, it uses continuously as a polycondensation reaction catalyst. Of course, it may be added in two or more times for the purpose of controlling the polycondensation reaction rate.

Next, the second reaction in which the precursor obtained in the first reaction is polycondensed will be described.
In the present invention, for the purpose of imparting a high degree of thermal stability to the obtained polyester, it is preferable to add a thermal stabilizer composed of a phosphorus compound to the reaction system before the start of the polycondensation reaction in the second reaction. The specific phosphorus compound is not particularly limited as long as it has a phosphorus element in the compound. For example, phosphoric acid, phosphorous acid, phosphoric acid trimethyl ester, phosphoric acid tributyl ester, phosphoric acid triphenyl ester, phosphorus Examples include acid monomethyl ester, phosphoric acid dimethyl ester, phenylphosphonic acid, phenylphosphonic acid dimethyl ester, phenylphosphonic acid diethyl ester, ammonium phosphate, triethylphosphonoacetate, methyldiethylphosphonoacetate, and these phosphorus compounds May use 2 or more types together. The addition timing of the phosphorus compound is preferably performed during the initial stage of the polycondensation reaction, which is the second reaction after the first reaction is substantially completed, and the addition may be performed at one time or two or more times. You may divide into.

  By the way, the polycondensation reaction is preferably performed at a temperature in the range of 270 ° C. to 300 ° C., and the pressure during the polycondensation reaction is preferably performed under a reduced pressure of 50 Pa or less. If the pressure during the polycondensation reaction is higher than the upper limit, the time required for the polycondensation reaction becomes long and it becomes difficult to obtain a copolymerized aromatic polyester having a high degree of polymerization. As the polycondensation catalyst, known metal compounds such as Ti, Al, Sb, and Ge can be suitably used. Among these, when Mn-Sb is used, the dispersibility of particles in the master polyester described later is improved. It is preferable because it is possible.

  In addition, as for the method of incorporating particles, as a slurry state of alkylene glycol, coarse particles are further reduced by a filter or the like, and added in the polymerization step, whereby particles having a particle content of 0.02 to 1.0% by weight. It is preferable to prepare a containing master polyester and dilute the master polyester with a polyester containing no particles in order to reduce coarse protrusions due to aggregation of particles.

  The polyester thus obtained contains a stabilizer such as an ultraviolet absorber, a lubricant such as an antioxidant, a plasticizer, and a wax, a flame retardant, a release agent, and a nucleating agent as long as the effects of the present invention are not impaired. You may mix | blend as needed. In addition, from the viewpoint of making the background index on the surface on the side where the magnetic layer is formed into a desired range, other thermoplastic polymers that are incompatible with polyester, pigments, fillers, glass fibers, carbon fibers, layered silicates, etc. Is preferably not contained.

  By the way, the biaxially oriented polyester film of the present invention has a surface index of 80 to 99.99% on at least one surface, preferably a surface with a flatr surface roughness, such as a surface forming a magnetic layer. It is a range. More preferably, it is in the range of 85 to 99.5%, particularly preferably in the range of 90 to 99.5%, and most preferably in the range of 96 to 99.5%. The background index is a value measured by a non-contact type three-dimensional surface roughness meter, and is a numerical value indicating the area ratio of the surface excluding protrusions and dents on the film surface. It is one of the features of the present invention that the background index is closely related to the electromagnetic conversion characteristics and the error rate after storage of the tape cartridge. When the background index is within the above range, it is possible to highly reduce the electromagnetic conversion characteristics and the error rate after storing the tape cartridge. In addition, when the film is processed at a high temperature, for example, in the step of applying and drying the magnetic layer, the background index is in the above range, so that the coating layer can be uniformly dried, Even if the film is processed at a high temperature at which the viscoelasticity of the film is lowered, the surface property change before and after the treatment can be suppressed because there are few protrusions that change at a high temperature. In order to make such a surface index within a desired range, it is effective to control the particles on the opposite side, which will be described later, preferably on the rough surface layer side having a rough surface roughness, as described later. In the production process of the biaxially oriented polyester film, it is very effective to stretch the film under the conditions described below. In order to increase the background index, it is sufficient to select conditions such as stretching at a temperature at which the viscoelasticity is as low as possible during transverse stretching of the film and at a relatively high temperature at which the crystal does not progress instantaneously at that temperature itself. On the other hand, if it is desired to reduce the stretching temperature, a condition for lowering the stretching temperature may be selected, such as stretching at a temperature at which a decrease in viscoelasticity starts during transverse stretching of the film.

  Further, the biaxially oriented polyester film of the present invention requires that the heat shrinkage rate at 130 ° C. in the width direction of the film is 3% or less. Preferably, it is 2.9% or less, more preferably 2.8% or less, particularly 2.3% or less. By having such a low thermal shrinkage rate while having a background index as described above, wrinkles in the coating process in a high-temperature environment can be reduced, and folding wrinkles at the time of conveyance roll contact can be reduced and conveyance performance can be achieved. Processability at the time of drying can be ensured, and it is possible to achieve a high balance between surface properties. On the other hand, the heat shrinkage rate at 130 ° C. in the longitudinal direction is also preferably 3% or less. More preferably, it is 2.9% or less, further preferably 2.8% or less, and particularly preferably 2.3% or less. It becomes possible to reduce wrinkles of a film that is tensioned in a high temperature environment such as a coating process.

  In the biaxially oriented polyester film of the present invention, at least one surface preferably has a surface with a flatr surface roughness, such as a surface on which a magnetic layer is formed, within a bearing drop of 10 to 30 nm. More preferably, it is 10-28 nm, Most preferably, it is the range of 10-25 nm. This bearing drop shows a steep curve drop seen in a region where the bearing area is 0.4% in a bearing curve created from surface data measured by a non-contact type three-dimensional surface roughness meter. . A large value indicates a large difference in height between the high and low protrusions, and a low value indicates that the same height protrusions are generally present uniformly. In particular, in a film used for magnetic tape, if there is a steep protrusion on the magnetic layer side, that is, if the bearing head is too large, the head and film spacing at the corresponding location will increase, resulting in electromagnetic conversion characteristics and dropout. cause. On the other hand, if this value is too small, there is only a small protrusion on the film, so that the transportability deteriorates. The above-mentioned range is preferable in order to have transportability and characteristics as a magnetic tape. In order to make such a bearing drop range, it is preferable to blend the particle size and concentration of the particles added to the film within the aforementioned range.

  For example, in the case of a laminated film, the biaxially oriented polyester film of the present invention is prepared by preparing a polyester polymer for a magnetic layer and a polyester polymer for forming an opposite surface, and laminating these in a molten state to form a sheet from a die. Manufactured by coextruding to the film, cooling and solidifying the obtained sheet-like material to form a laminated unstretched polyester film, and stretching the obtained laminated unstretched polyester film in the film forming direction and the width direction it can. The temperature in the process of extruding in a molten state is not particularly limited as long as there is no unmelted material and the temperature of the polyester does not excessively deteriorate. For example, the melting point (Tm: ° C.) to (Tm + 70) ° C. It is preferable to carry out at temperature. Next, for cooling, in order to reduce the thickness unevenness while maintaining the flatness of the obtained laminated unstretched polyester film, a rotating cooling drum installed below the die along the film forming direction is used. It is preferable to cool the sheet-like material in close contact with it. Subsequently, for stretching, the laminated unstretched polyester film is uniaxially (longitudinal or transverse) (polyester glass transition temperature (Tg) −10) ° C. to (Tg + 60) ° C. to 2.5 times or more. Preferably, the film is stretched at a magnification of 3 times or more, and then stretched at a temperature of Tg to (Tg + 60) ° C. at a temperature of 2.5 times or more, preferably at a magnification of 3 times or more in the direction orthogonal to the stretching direction. At this time, in order to keep the above-mentioned background index within a desired range, it is desirable that the transverse stretching temperature is stretched in the range of (Tg + 25) to (Tg + 60 ° C.). More preferred is (Tg + 30) to (Tg + 60 ° C.), particularly preferred is (Tg + 30) to (Tg + 55 ° C.), and most preferred is a range of (Tg + 35) to (Tg + 55 ° C.). At this time, the transverse stretching temperature is preferably increased stepwise, and any temperature is preferably within the above range. If the transverse stretching temperature is too low with respect to Tg, excessive stretching stress concentrates on the particles, and as a result, the voids around the particles become larger and the protrusions are higher and larger. On the other hand, when mildly stretched in the temperature range described above, the rough surface layer side can be flattened by simultaneously increasing the transverse stretch ratio higher than usual, and as a result, it has a desired height and size. Protrusions can be formed.

  Further, if necessary, the film may be stretched again in the machine direction and / or the transverse direction. The total draw ratio when stretched in this way is preferably 9 times or more, more preferably 12 to 35 times, and particularly preferably 15 to 30 times as an area draw ratio (longitudinal draw ratio x transverse draw ratio). . Furthermore, the biaxially oriented film can be heat-set at a temperature of (Tm-70) to (Tm-10) ° C., and is preferably heat-set at, for example, 180 to 250 ° C. The heat setting time is preferably 0.1 to 60 seconds. Moreover, although the above-mentioned extending | stretching was demonstrated by sequential biaxial stretching, you may use simultaneous biaxial stretching which extends | stretches simultaneously in the vertical direction and a horizontal direction.

  Further, the biaxially oriented polyester film of the present invention may be relaxed in the width direction while being heat-set or after heat-set. By relaxing in the width direction in this way, the heat shrinkage rate in the width direction of the film can be maintained in an appropriate range. This relaxation itself can also take place in the longitudinal direction. On the other hand, if relaxation is performed, the Young's modulus of the film is lowered, and a desired Young's modulus cannot be ensured, and tension may not be applied during processing. Therefore, the appropriate relaxation rate strongly depends on the polymer type of the film and the film forming conditions. For example, in the case of forming a polyethylene-2,6-naphthalate film, the relaxation temperature is set to 190 ° C. It is preferable to form a film at 3%.

  The biaxially oriented polyester film of the present invention preferably has a Young's modulus in the longitudinal direction of 5 GPa or more in order to exhibit excellent dimensional stability when used as a base film of a high-density magnetic recording medium. If the Young's modulus in the longitudinal direction is lower than that described above, the film tends to be stretched when tension is applied in the longitudinal direction during film handling, resulting in a problem. On the other hand, although there is no restriction | limiting about an upper limit, the higher one is preferable from a viewpoint of the said handling. The Young's modulus in the width direction is 4 to 15 GPa, more preferably 5 to 14 GPa, particularly 6 to 13 GPa, and most preferably 7 to 11 GPa from the viewpoint of easily setting the temperature expansion coefficient of the base film to the range described later. preferable. If the Young's modulus in the width direction is less than the lower limit, it becomes difficult to reduce the temperature expansion coefficient when it is used as a magnetic recording tape, or wrinkles are likely to enter the film with respect to the conveyance tension in the coating process. On the other hand, when the upper limit is exceeded, the temperature expansion coefficient when the magnetic recording tape is formed becomes excessively small.

  The total thickness of the biaxially oriented polyester film of the present invention is preferably 2.0 μm or more and 8.0 μm or less. The lower limit of the preferable total thickness is 2.5 μm, and further 3 μm. The upper limit of the preferable total thickness is 7 μm, further 6 μm, particularly 4.5 μm. When the thickness is smaller than the lower limit, the tape loses its elasticity, so that the electromagnetic conversion characteristics are deteriorated and wrinkles are easily formed in the coating process. When the thickness exceeds the upper limit, the tape length per one tape is shortened, so that it is difficult to reduce the size and increase the capacity of the magnetic tape.

  By the way, it is preferable that the biaxially oriented polyester film of the present invention is a laminated film from the viewpoint of further improving running properties and winding properties. A layer having a flat surface with a smaller surface roughness is referred to as A layer, and a layer having a rough surface with a larger surface roughness is referred to as B layer.

  When the biaxially oriented polyester film of the present invention is a laminated film, the aforementioned A layer satisfies the surface roughness (RaA), background index, and bearing curve, while the B layer has a surface roughness (RaB) of 2. It is preferable to be in the range of 0.0 to 6.0 nm. The lower limit of the surface roughness (RaB) of the preferred B layer is 2.2 nm, further 2.5 nm, particularly 4.0 nm, and the upper limit is 5.8 nm. If the surface roughness (RaB) of the B layer is lower than the lower limit of this range, the B layer is too flat, causing problems in handling the film. On the other hand, if it is rougher than the above range, the protrusion on the surface of the B layer forming the surface roughness of the B layer is pushed up, which affects the A layer. From such a viewpoint, when used for a base film of a magnetic tape, it is preferable to form a magnetic layer on the surface of the A layer.

  By the way, from the viewpoint of further improving the running property and winding property, the B layer preferably contains inert particles having an average particle diameter of 0.05 μm or more larger than the inert particles contained in the A layer. Particularly preferred are those containing the same inert particles contained in the A layer and inert particles having an average particle diameter of 0.05 μm or more larger than the inert particles contained in the A layer. . The preferable average particle diameter of the inert particles contained in the B layer and having an average particle diameter larger than that of the inert particles contained in the A layer is 0.15 to 0.25 μm, and further 0.16 to 0.24 μm. In addition, it can be said that the kind and particle size distribution of the inert particles contained in the B layer are the same as described above.

  In the present invention, the thickness ratio (tA / tB) of the A layer and the B layer is preferably in the range of 0.33 to 9.0. Preferred lower limits are 0.5, further 0.6, even more 0.8, in particular 2.0. A preferred upper limit is 8.0, further 7.0, even 6.0, especially 5.0. When the amount is less than the lower limit, the particles contained in the B layer on the rough surface layer side where the magnetic layer is not formed are pushed up, and electromagnetic conversion characteristics are easily deteriorated. On the other hand, when the amount is equal to or more than the above upper limit, the lubricant of the rough surface layer that does not form the magnetic layer is easy to drop off, and the resin that is usually recovered can be reused because it can be used only for the B layer so as not to impair the flatness. Problems such as an extremely low rate occur.

  Furthermore, the biaxially oriented polyester film of the present invention has a thickness tA (μm) of the A layer and an average particle diameter of the inert particles having the largest average particle diameter among the inert particles added to the B layer (DpB: μm). ) Ratio (tA / DpB) is preferably 15 or more. If it is less than the above lower limit, the particles in the B layer that do not form the magnetic layer are pushed up to the magnetic layer, and the flatness tends to be deteriorated. On the other hand, the upper limit is not particularly limited as long as a desired film can be obtained. Preferred tA / DpB is 15 to 25, more preferably 15 to 22.

  The biaxially oriented polyester film of the present invention is preferably used as a base film for high-density magnetic recording tapes, particularly digital recording magnetic recording tapes. Therefore, the magnetic recording medium using the biaxially oriented polyester film of the present invention will be further described.

  The magnetic recording medium of the present invention can be produced by forming a magnetic layer on the above-mentioned biaxially oriented polyester film. It should be noted that the surface of the biaxially oriented polyester film of the present invention has a well-known easy adhesion function as long as the effect of the present invention is not impaired in order to improve the adhesion with a magnetic layer or the like. Etc. may be formed.

  The magnetic layer in the magnetic recording tape of the present invention is prepared by uniformly dispersing iron or acicular fine magnetic powder or barium ferrite in a binder such as polyvinyl chloride or a vinyl chloride / vinyl acetate copolymer. A magnetic recording tape that is formed by applying a liquid, and as described above, is excellent in electromagnetic conversion characteristics and error rate performance while maintaining processability by using the biaxially oriented polyester film of the present invention. It can be.

  By the way, as described above, in order to increase the recording density, it is necessary to make the magnetic material finer. Therefore, it becomes difficult to remove the solvent from the coating liquid, and if the workability is maintained, drying or the like is performed at a higher temperature. It was necessary to do in. And when it was going to process the film which has a very flat surface at high temperature, there existed new problems, such as wrinkles, and it reached | attained this invention.

  In addition, it is excellent in electromagnetic conversion characteristics such as output in a short wavelength region, S / N, C / N, etc., particularly when the magnetic layer is applied so that the thickness is 1 μm or less, and further 0.1 to 1 μm. This is preferable from the viewpoint of a coating type magnetic recording tape for high density recording with low dropout and error rate. If necessary, it is also preferable to disperse and coat a nonmagnetic layer containing fine titanium oxide particles or the like in the same organic binder as that of the magnetic layer as the underlayer of the coating type magnetic layer.

Further, a protective layer such as diamond-like carbon (DLC) and a fluorine-containing carboxylic acid-based lubricating layer are sequentially provided on the surface of the magnetic layer as required, and a known backcoat layer is provided on the other surface. May be provided.
The coating type magnetic recording tape thus obtained is extremely useful as a magnetic tape for data use such as data 8 mm, DDSIV, DLT, S-DLT, LTO and the like.

  Hereinafter, the present invention will be described more specifically with reference to examples and comparative examples. In addition, the characteristic of the polyester in this invention, a biaxially-oriented polyester film, and data storage was measured and evaluated by the following method.

(1) Intrinsic viscosity As described above, the intrinsic viscosity of the obtained polyester is measured at o-chlorophenol at 35 ° C. When it is difficult to dissolve uniformly with o-chlorophenol, p-chlorophenol / It was determined by measurement at 35 ° C. using a mixed solvent of 1,1,2,2-tetrachloroethane (40/60 weight ratio).

(2) Particle size of particles in the film The polyester in the film surface layer is removed by a plasma low-temperature ashing method (for example, PR-503, manufactured by Yamato Kagaku) to expose the particles. The treatment conditions are such that the polyester is ashed but the particles are not damaged. This is observed with a SEM (scanning electron microscope) at a magnification of about 10,000 times, and an image of the particle (light density produced by the particle) is connected to an image analyzer (for example, QTM900, manufactured by Cambridge Instrument) The observation area is changed, and the area equivalent circle diameter (Di) of at least 5,000 particles is obtained. From this result, a particle size distribution curve of the particles was prepared. The identification of the particle type can be performed using SEM-XMA, quantitative analysis of metal elements by ICP, or the like. Moreover, the average particle diameter of the inert particle to add was measured similarly, the particle diameter of each particle | grain was calculated | required, and the number average was made into the average particle diameter.

(3) Content of particles (3-1) Total content of particles in each layer The polyester A layer and the polyester B layer were sampled by scraping about 100 g each from the laminated biaxially oriented polyester film, the polyester was dissolved, and the particles were After selecting the solvent not to be dissolved and dissolving the sample, the particles are centrifuged from the polyester, and the ratio of the particles to the sample weight (% by weight) is the total particle content in each layer.

(3-2) Total content of inorganic particles in each layer When inorganic particles of the laminated polyester film are present, the polyester A layer and the polyester B layer are each scraped and sampled about 100 g, and this is sampled in a platinum crucible. Burn in a furnace at about 1,000 ° C. for 3 hours or more, then mix the burned product in the crucible with terephthalic acid (powder) to make a 50 gram tablet plate. This plate is converted using a wavelength-dispersed fluorescent X-ray to calculate the count value of each element from a calibration curve for each element that has been prepared in advance, and the total content of inorganic particles in each layer is determined. The X-ray tube for measuring fluorescent X-rays is preferably a Cr tube and may be measured with an Rh tube. The X-ray output is set to 4 kW, and the spectral crystal is changed for each element to be measured. When there are a plurality of types of inorganic particles of different materials, the content of the inorganic particles of each material is determined by this measurement.

(3-3) Content of various particles in each layer (when no inorganic particles are present)
When inorganic particles are not present in the layer, the weight ratio of the particles present in each peak region is calculated from the number ratio of each particle constituting the peak determined by the above (2), the average particle diameter, and the density of the particles, From this and the total content of particles in each layer determined in (3-1) above, the content (% by weight) of particles present in each peak region is determined.
The typical fine particle density is as follows.
Density of crosslinked silicone resin: 1.35 g / cm ³
Cross-linked polystyrene resin density: 1.05 g / cm ³
Cross-linked acrylic resin density: 1.20 g / cm ³
The resin density is further determined by separating the particles centrifuged from the polyester by the method (3-1), and measured by a method described in “Fine Particles Handbook: Asakura Shoten, 1991 edition, page 150”, for example, with a pycnometer. be able to.

(3-4) Content of various particles in each layer (when inorganic particles are present)
When inorganic particles are present in the layer, the layer is determined from the total content of particles in each layer determined in (3-1) and the total content of inorganic particles in each layer determined in (3-2). The content of the organic particles and the inorganic particles in each is calculated, the content of the organic particles is the method (3-3), and the content of the inorganic particles is the method (3-2). (Wt%) is determined.

(4) Thickness of film and each polyester layer (4-1) Thickness of film 10 films are stacked so that dust does not enter, the thickness is measured with a dot-type electronic micrometer, and the film thickness per sheet is calculated. To do.

(4-2) Thickness of each polyester layer Using a secondary ion mass spectrometer (SIMS), an element caused by the highest concentration of particles in the film ranging from the surface layer to a depth of 3,000 nm The concentration ratio (M + / C +) of the carbon element in the polyester is defined as the particle concentration, and analysis in the thickness direction is performed from the surface to a depth of 3,000 nm. In the surface layer, the particle concentration is low due to the interface of the surface, and the particle concentration increases as the distance from the surface increases. And the particle concentration once reached the maximum value starts to decrease again. Based on this concentration distribution curve, a depth at which the surface layer particle concentration is ½ of the maximum value (this depth is deeper than the depth at which the maximum value is reached) is determined, and this is defined as the surface layer thickness. Then, the thickness of each layer is calculated from the thickness of the film and the thickness of the surface layer.
The conditions are as follows.
(A) Measuring device: secondary ion mass spectrometer (SIMS)
(B) Measurement conditions Primary ion species: O ²⁺
Primary ion acceleration voltage: 12KV
Primary ion current: 200 nA
Raster area: 400 μm
Analysis area: 30% gate
Measurement degree of vacuum: 0.8 Pa (6.0 × 10 ⁻³ Torr)
E-GUN: 0.5KV-3.0A
In addition, when the most contained particles in the range from the surface layer to a depth of 3000 nm are organic polymer particles, it is difficult to measure by SIMS, so XPS (X-ray photoelectron spectroscopy), IR (infrared spectroscopy) while etching from the surface. The depth profile similar to the above may be measured by the method) to obtain the surface layer thickness.

(5) Young's modulus The film is cut into a sample width of 10 mm and a length of 15 cm, and the distance between chucks is set to 100 mm, and the film is pulled with an Instron type universal tensile tester under the conditions of a tensile speed of 10 m / min and a chart speed of 500 mm / min. The Young's modulus is calculated from the tangent of the rising portion of the obtained load-elongation curve.

(6) Surface roughness (Ra)
Measured using a non-contact type three-dimensional surface roughness meter (manufactured by ZYGO: New View 5022) at a measurement magnification of 25 times and a measurement area of 283 μm × 213 μm (= 0.0603 mm ² ), and incorporated in the roughness meter The center surface average roughness (Ra) was determined by the surface analysis software MetroPro, which was defined as the surface roughness (Ra). The measurement was performed 10 times while changing the measurement location, and the average value thereof was defined as the center plane average roughness (Ra). The surface roughness of the flat side (A layer side) of the laminated polyester film was RaA, and the surface roughness of the rough side (B layer side) was RaB.

(7) Background Index After measuring Ra under the condition (6) above using a non-contact type three-dimensional surface roughness meter (manufactured by ZYGO: New View 5022), the software Metro Pro built in the roughness meter By drawing a line that is 5 nm away from the center line on the surface in the height direction on the convex side and the concave side, respectively, a thing having a height higher than that is recognized as a protrusion, and further has an area of 0.5 μm ² or more The protrusions were counted as the number of protrusions. The projection areas of all the projections were totaled, and a value obtained by subtracting the measurement area 283 μm × 213 μm = (0.0603 mm ² ) as a percentage of the measurement area was determined as the background index in the present invention.

(8) Bearing drop After measuring Ra under the condition (6) above using a non-contact type three-dimensional surface roughness meter (manufactured by ZYGO: New View 5022), the roughness data is analyzed with software for analysis (Image Metrology). (Made by SPIP), a bearing curve was created, and the maximum value of the steep drop of the bearing curve where the bearing area in this bearing curve is 0.4% was quantified as the bearing drop.

(9) Coefficient of static friction of the film An acrylic plate fixed on the lower side of two films (each 20 cm in the vertical direction × 10 cm in the horizontal direction) on which the surface on the polyester A layer side and the surface on the polyester B layer side were placed, A thread is placed at the center of the upper side of the two overlapped films, fixed to the acrylic plate with the surface with the larger surface roughness facing down, and the acrylic plate is taken up with a low speed roll (10 cm). / Min), a detector is fixed to one end of the upper film (opposite to the take-off direction of the lower film), and the tensile force at the start of the film / film is detected. The thread used at that time has a weight of 200 g and a lower area of 50 cm ² (rectangle of 10 cm in the vertical direction × 5 cm in the horizontal direction).
The static friction coefficient (μs) was obtained from the following equation.
μs = (Tensile force at start g) / (Load 200 g)
When the static friction coefficient of the film increases, slipperiness decreases, and wrinkles and defects are likely to occur when the film is wound into a roll.

(10) Rate of non-defective product taken as follows according to the ratio of non-blocking and wrinkle-free good products when 100 rolls are taken at a length of 5000 m while slitting a 1 m wide product roll from the parent roll at 95 m / min. To do.
◎; Percentage without defects such as blocking and wrinkles 85-100%
○: Same as above 70-84%
×: Same as above, less than 70%

(11) Heat treatment wrinkle test A film having a width of 10 cm and a length of 25 cm was prepared as a test piece, and heat treatment was performed at 130 ° C for 1 minute in a state where a load of 1 kg / m was applied. The elongation in the longitudinal direction of the heat-treated film and the number of wrinkles generated in the longitudinal direction were measured in the width direction.

(12) Preparation of magnetic tape A back coat layer paint having the following composition was applied to the surface of the rough surface layer (A layer) of the laminated biaxially oriented polyester film having a width of 1000 mm and a length of 1000 m obtained in each Example and Comparative Example. After coating and drying with a die coater (tension during processing: 20 MPa, temperature: 130 ° C., speed: 200 m / min), a non-magnetic paint or magnetic paint having the following composition on the surface of the flat layer (B layer) side of the film Is applied at the same time with a die coater while changing the film thickness, magnetically oriented and dried. Further, after calendering with a small test calender (steel roll / nylon roll, 5 stages) at a temperature of 70 ° C. and a linear pressure of 200 kg / cm, curing is performed at 70 ° C. for 48 hours. The tape was slit to 12.65 mm and incorporated into a cassette to obtain a magnetic recording tape. The coating amount was adjusted so that the thicknesses of the dried backcoat layer, nonmagnetic layer and magnetic layer were 0.5 μm, 1.2 μm and 0.1 μm, respectively.
<Composition of non-magnetic paint>
-Titanium dioxide fine particles: 100 parts by weight-ESREC A (vinyl chloride / vinyl acetate copolymer made by Sekisui Chemical: 10 parts by weight)-Nipporan 2304 (polyurethane elastomer made by Nippon Polyurethane): 10 parts by weight-Coronate L (polyisocyanate made by Nippon Polyurethane ): 5 parts by weight-lecithin: 1 part by weight-methyl ethyl ketone: 75 parts by weight-methyl isobutyl ketone: 75 parts by weight-toluene: 75 parts by weight-carbon black: 2 parts by weight-lauric acid: 1.5 parts by weight <magnetic paint Composition>
Iron (major axis: 0.025 μm, needle ratio: 3.5, 2350 oersted): 100 parts by weight Eslek A (vinyl chloride / vinyl acetate copolymer made by Sekisui Chemical: 10 parts by weight) Nipponan 2304 (Nippon Polyurethane Polyurethane elastomer): 10 parts by weight, Coronate L (polyisocyanate made by Nippon Polyurethane): 5 parts by weight, lecithin: 1 part by weight, methyl ethyl ketone: 75 parts by weight, methyl isobutyl ketone: 75 parts by weight, toluene: 75 parts by weight, carbon Black: 2 parts by weight ・ Lauric acid: 1.5 parts by weight <Composition of back coat layer paint:>
Carbon black: 100 parts by weight Thermoplastic polyurethane resin: 60 parts by weight Isocyanate compound: 18 parts by weight (Coronate L manufactured by Nippon Polyurethane Industry Co., Ltd.)
Silicone oil: 0.5 parts by weight Methyl ethyl ketone: 250 parts by weight Toluene: 50 parts by weight

(13) Electromagnetic conversion characteristics For measuring the electromagnetic conversion characteristics, a 1/2 inch linear system with a fixed head was used. Recording was performed using an electromagnetic induction head (track width 25 μm, gap 0.1 μm), and reproduction was performed using an MR head (8 μm). The relative speed of the head / tape is 10 m / sec, a signal with a recording wavelength of 0.2 μm is recorded, the reproduced signal is analyzed with a spectrum analyzer, the output C of the carrier signal (wavelength 0.2 μm), and the integration over the entire spectrum. A relative value with the noise N ratio as C / N ratio and Example 2 as 0 dB was determined and evaluated according to the following criteria.
◎: +1 dB or more ○: −1 dB or more, less than +1 dB ×: less than −1 dB

(14) Error rate Data storage with a length of 850 m of magnetic recording tape by slitting the original tape produced in (11) above to a width of 12.65 mm (1/2 inch) and incorporating it into an LTO case. A cartridge was created. This data storage was recorded using an IBM LTO5 drive in an environment of 23 ° C. and 50% RH (recording wavelength 0.55 μm), and then the cartridge was stored in an environment of 50 ° C. and 80% RH for 7 days. After the cartridge was stored at room temperature for one day, the full length was reproduced, and the error rate of the signal at the time of reproduction was measured. The error rate is calculated from the error information (number of error bits) output from the drive by the following formula. The dimensional stability is evaluated according to the following criteria.
Error rate = (number of error bits) / (number of write bits)
A: Error rate is less than 1.0 × 10 ⁻⁶ ○: Error rate is 1.0 × 10 ⁻⁶ or more, less than 1.0 × 10 ⁻⁴ ×: Error rate is 1.0 × 10 ⁻⁴ or more

(15) Dropout (DO)
The data storage cartridge whose error rate was measured in the above (14) was loaded into an IBM LTO5 drive, a data signal was recorded at 14 GB, and it was reproduced. A signal having an amplitude (PP value) of 50% or less with respect to the average signal amplitude was detected as a missing pulse, and four or more consecutive missing pulses were detected as dropouts. In addition, dropout evaluates 1 volume of 850m, converts into the number per 1m, and determines by the following references | standards.
◎: Dropout less than 3 pieces / m ○: Dropout of 3 pieces / m or more, less than 9 pieces / m ×: Dropout of 9 pieces / m or more

[Example 1]
As particles to be added to the flat layer side, the intrinsic viscosity containing 0.01% by weight of true spherical silica particles (particles A) having an average particle size of 0.08 μm (relative standard deviation of particle size: 10%) is 0.62. Polyethylene-2,6-naphthalate pellets for polyester A layer (glass transition temperature: 121 ° C., melting point: 265 ° C.) and particles to be added to the rough surface layer side, an average particle size of 0.08 μm (relative standard deviation of particle size) : 10%) of true spherical silica particles (particle B1) of 0.35% by weight and 0.03% of true spherical silica (particles B2) having an average particle size of 0.2 μm (relative standard deviation of particle size: 10%). %, Polyethylene-2,6-naphthalate pellets (glass transition temperature: 121 ° C., melting point: 265 ° C.) for polyester B layer having an intrinsic viscosity of 0.62. Each pellet was dried at 170 ° C. for 6 hours and then supplied to two extruder hoppers, respectively, at a melting temperature of 310 ° C., and a thickness ratio of A layer: B layer = 75: 25 from the die onto the cooling drum. The sheet was coextruded to obtain a laminated unstretched polyester film.
The laminated unstretched polyester film thus obtained was preheated to 120 ° C. and heated from above with an IR heater so that the film surface temperature was 140 ° C. Stretching in the film direction) was performed. Subsequently, it is supplied into a stenter heated to 155 ° C., and is stretched 5.3 times in the lateral direction (first stage) while being gradually increased to 165 ° C. and 170 ° C., and further heated to 180 ° C. After being fed into the stenter and stretched 1.2 times in the transverse direction, it was heat-fixed with hot air at 215 ° C for 4 seconds, and then relaxed in the width direction at 190 ° C and a relaxation rate of 0.27%. Thus, a laminated biaxially oriented polyester film having a thickness of 4.0 μm was obtained. The Young's modulus of the obtained laminated biaxially oriented polyester film was 6.5 GPa in the vertical direction and 8.9 GPa in the horizontal direction. The background index of the polyester A layer was 98.38%.
Table 1 shows the characteristics of the obtained laminated biaxially oriented polyester film.

[Example 2]
A laminated biaxially oriented polyester film was obtained in the same manner as in Example 1 except that the transverse stretching temperature was changed to 124, 128, and 158 ° C. so as to change the temperature stepwise.
Table 1 shows the characteristics of the obtained laminated biaxially oriented polyester film.

[Examples 3 to 4]
The same operation as in Example 1 was repeated except that the thicknesses of the particles A, particles B1, and particles B2 and the respective layers to be contained were changed as shown in Table 1. Table 1 shows the characteristics of the obtained laminated biaxially oriented polyester film.

[Example 5]
Polyethylene terephthalate pellets for polyester A layer containing 0.01% by weight of true spherical silica particles (particle A) having an average particle size of 0.08 μm as particles to be added to the flat layer side (glass) Transition temperature: 76 ° C., melting point: 255 ° C.) and polyetherimide pellets (trade name: ULTEM 1040) blended at a weight ratio of 90:10, and the average particle size as particles added to the rough surface layer side Polyester having an intrinsic viscosity of 0.62 containing 0.35% by weight of true spherical silica particles (particle B1) of 0.08 μm and 0.03% by mass of true spherical silica (particles B2) having an average particle diameter of 0.20 μm B layer polyethylene-terephthalate pellets (glass transition temperature: 76 ° C, melting point: 255 ° C) and polyetherimide pellets (trade name: ULTEM1) 40) and a resin composition blended at a weight ratio of 90:10, respectively, dried in a pellet state at 170 ° C. for 3 hours, and then supplied to two extruder hoppers, respectively. : B layer = 77: 23 The thickness ratio was coextruded from a die onto a cooling drum in a sheet shape to obtain a laminated unstretched polyester film.
The laminated unstretched polyester film thus obtained was preheated to 75 ° C. and heated from above with an IR heater so that the film surface temperature was 90 ° C., and stretched in the machine direction (manufactured at a stretch ratio of 4.8 times). Stretching in the film direction) was performed. Subsequently, it was supplied into a stenter heated to 90 ° C., and while being stepwise raised to 125 ° C. and 130 ° C., it was stretched 5 times in the lateral direction (first stage) and further heated to 180 ° C. After being supplied into the stenter and stretched 1.2 times in the transverse direction again, it was heat-fixed with hot air at 230 ° C for 4 seconds, and then relaxed at 210 ° C with a relaxation rate of 1%, and then laminated with a thickness of 4.5 µm A biaxially oriented polyester film was obtained. The Young's modulus of the obtained laminated biaxially oriented polyester film was 4.9 GPa in the vertical direction and 7.6 GPa in the horizontal direction.
Table 1 shows the characteristics of the obtained laminated biaxially oriented polyester film.

[Comparative Examples 1 to 7]
The same operation as in Example 1 was repeated except that the particle A, particle B1, particle B2, the thickness of each layer, the heat setting temperature, and the relaxation rate were changed as shown in Table 1. Table 1 shows the characteristics of the obtained laminated biaxially oriented polyester film.

[Comparative Example 8]
Polyethylene-terephthalate pellets for polyester A layer having an intrinsic viscosity of 0.62 containing 0.01% by weight of true spherical silica particles (particle A) having an average particle size of 0.08 μm as the particles added to the flat layer side (glass transition (Temperature: 76 ° C., melting point: 255 ° C.) As particles to be added to the rough surface layer side, 0.35% by weight of true spherical silica particles (particle B1) having an average particle size of 0.08 μm and an average particle size of 0.20 μm Polyethylene-terephthalate pellets (glass transition temperature: 76 ° C., melting point: 255 ° C.) for polyester B layer having an intrinsic viscosity of 0.62 and containing 0.03% by mass of true spherical silica (particle B2) were prepared. Each pellet was dried at 170 ° C. for 3 hours, and then supplied to two extruder hoppers. At a melting temperature of 280 ° C., a thickness ratio of A layer: B layer = 77: 23 from the die onto the cooling drum. The sheet was coextruded to obtain a laminated unstretched polyester film.
The laminated unstretched polyester film thus obtained was preheated to 75 ° C. and heated from above with an IR heater so that the film surface temperature was 90 ° C., and stretched in the machine direction (manufactured at a stretch ratio of 4.8 times). Stretching in the film direction) was performed. Subsequently, it was supplied into a stenter heated to 90 ° C., and while being stepwise raised to 125 ° C. and 130 ° C., it was stretched 5 times in the lateral direction (first stage) and further heated to 180 ° C. After being supplied into the stenter and stretched 1.2 times in the transverse direction again, it was heat-fixed with hot air at 230 ° C for 4 seconds, and then relaxed at 210 ° C with a relaxation rate of 1%, and then laminated with a thickness of 4.5 µm A biaxially oriented polyester film was obtained. The Young's modulus of the obtained laminated biaxially oriented polyester film was 4.9 GPa in the vertical direction and 7.6 GPa in the horizontal direction.
Table 1 shows the characteristics of the obtained laminated biaxially oriented polyester film.

  In Table 1, silica means true spherical silica particles, PEN means polyethylene-2,6-naphthalene dicarboxylate, PET means polyethylene terephthalate, and PEI means polyetherimide.

  The biaxially oriented laminated polyester film of the present invention is excellent in productivity and also has workability such as subsequent transportability, excellent electromagnetic conversion characteristics, and a coating type magnetic recording tape with reduced error rate and dropout, particularly It can be suitably used as a base film for data storage.

Claims (10)

At least one surface contains 50 ppm (mass basis) or more of inert particles having an average particle diameter of 0.05 to 0.2 μm, the surface roughness (RaA) is 1.5 nm or less, and the background index is 80 to 99. A biaxially oriented polyester film having a bearing curve drop of 10-30 nm in a range of .99%;
The biaxially oriented polyester film, wherein the biaxially oriented polyester film has a heat shrinkage rate at 130 ° C. of 3% or less in the film width direction.   The biaxially oriented polyester film according to claim 1, wherein the polyester mainly comprises ethylene terephthalate or ethylene-2,6-naphthalenedicarboxylate as a repeating unit.   The biaxially oriented polyester film according to claim 1, which has a thickness of 2.0 µm or more and 8.0 µm or less.   The biaxially oriented polyester film according to claim 1, wherein the polyester film has at least two layers and the other surface has a surface roughness (RaB) of 2.0 nm or more and 6.0 nm or less.   The ratio (tA / tB) of the thickness tA (μm) of the layer having a flat surface having a smaller surface roughness to the thickness tB (μm) of the layer having a rough surface having a larger surface roughness is 0.5 or more. The biaxially oriented polyester film according to claim 4, which is 9 or less.   A layer having a rough surface with a larger surface roughness is 0.05 μm or more inactive with a larger average particle size than the average particle size of the inert particles contained in a layer having a flat surface with a smaller surface roughness. The thickness tA (μm) of the layer containing particles and having a flat surface with a smaller surface roughness, and the maximum particle diameter DpB of the inert particles added to the layer having a rough surface with a larger surface roughness ( The biaxially oriented polyester film according to claim 4, wherein a ratio (tA / DpB) of μm) is 15 or more.   The biaxially oriented polyester film according to claim 1, wherein the heat shrinkage rate at 130 ° C in the film forming direction is 3% or less.   The biaxially oriented polyester film according to claim 1, wherein the Young's modulus in the film forming direction is 5 GPa or more.   The biaxially oriented polyester film according to claim 1, wherein the inert particles contained are any of spherical silica particles, crosslinked polystyrene particles, silicone particles, and silica-acryl composite particles.   A coated magnetic recording tape comprising the biaxially oriented polyester film according to any one of claims 1 to 9 and a magnetic layer coated and formed on a surface on which the magnetic layer is formed.